Japanese Patent Application No. 2000-182135, filed Jun. 16, 2000, is hereby incorporated by reference in its entirety.
The present invention relates to a surface-emitting device using electroluminescence.
An electroluminescence light-emitting element using electroluminescence has the problem that since the light is emitted isotropically and the directivity is poor, when viewed in a particular direction the light is of low strength, and light emission of high efficiency is not possible.
The object of the present invention is the provision of a surface-emitting device capable of increasing the power of emitting light in a particular direction, and of making efficient use of the light.
The surface-emitting device of the present invention comprises a substrate and a light-emitting device section formed on the substrate, and emitting light in a direction intersecting the substrate,
wherein the light-emitting device section includes:
a light-emitting layer capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electrical field to the light-emitting layer; and
a grating of at least second order.
According to the surface-emitting device of the present invention, from the pair of electrode layers, that is, the cathode and anode, electrons and holes respectively are injected into the light-emitting layer, these electrons and holes recombine in the light-emitting layer, and when the molecules return from the excited state to the ground state, light is generated. According to the surface-emitting device of the present invention, since a grating of at least second order is included, light generated by the emitting layer is controlled by the grating to be emitted in a direction intersecting the substrate.
Here, a grating of at least second order refers to a grating, for example, such that if the pitch in the periodic direction of the grating is (a+1) xcex/2n (xe2x80x9caxe2x80x9d is a positive integer, xe2x80x9cnxe2x80x9d is the mean refractive index). In particular, in the case of a grating of second order (axe2x88x921), a surface-emitting device can be obtained in which light is emitted not only in the direction of extension of the substrate 10 (the X-direction in FIG. 1), but in the film thickness direction of the substrate 10 (the Y-direction in FIG. 1).
A grating refers to an optical element generally used to obtain a particular spectrum using the diffraction of light.
In this case, in the above-described surface-emitting device of the present invention, the grating may form a photonic band gap or a photonic band approximating to the photonic band gap. Here a photonic band approximating to a photonic band gap refers to a band formed when a complete photonic band gap is not formed. For example, when the grating is formed by alternating arrays of the first medium layers and second medium layers, the photonic band gap may not be completely formed in the case where the difference in refractive index between the first medium layer and second medium layer is small.
According to this construction, from the pair of electrode layers, that is, the cathode and anode, electrons and holes respectively are injected into the light-emitting layer, these electrons and holes recombine in the light-emitting layer, and when the molecules return from the excited state to the ground state, light is generated. That is to say, within the light-emitting layer, the recombination of these electrons and holes generates excitons, and on the deactivation of these excitons, light in the form of fluorescence, phosphorescence, or the like is generated. By means of this, light of an extremely narrow emission spectral range can be obtained with high efficiency, and restricted spontaneous emission.
As examples of a surface-emitting device including the above described grating may be cited the following first and second surface-emitting devices.
In the first surface-emitting device according to the above-described surface-emitting device of the present invention, the grating may be formed so that an energy level of an emission spectrum of the light-emitting layer includes a band edge energy level included within a band formed by the grating.
According to the first surface-emitting device, a band with respect to light is formed by the grating. This band provides a high state density with a given band edge energy. Here, the grating is constructed so that in the light-emitting layer, the energy level of the spectrum of the emitted light includes this band edge energy level, and therefore, in the light emitting layer the light emission at this band edge energy level is more easily attained. By means of this, light including the wavelength corresponding to this band edge energy level, and of a narrow spectral band is emitted, and an element with a high efficiency can be obtained.
The second surface-emitting device may comprise a defect formed in a part of the grating, and so that the energy level arising from the defect is within a given emission spectrum. According to this construction, of the light generated in the light-emitting layer, only the light of the wavelength band corresponding to the energy level arising from the defect can be propagated within the grating. Therefore, by determining the energy level width arising from the defect, light with natural emission regulated in a given direction, with an extremely narrow emission spectral range, and with directivity can be obtained with a high yield.
In this case, the light-emitting layer can function as at least part of the grating. In this case, the light-emitting layer may function as at least part of the defect.
In the above-described first and second surface-emitting devices, for example, the following configuration are possible.
(1) The grating may be a distributed feedback type grating, or a distributed Bragg reflection type, and further, may have a gain coupled structure or a refractive index coupled structure.
(2) The grating may have a first medium layer and a second medium layer disposed periodically and being insulated, and may have a periodic refractive index distribution in at least one direction. For example, there may be periodicity in one direction, in given two directions (first and second directions), or in given three directions (first, second, and third directions).
In this case, the surface-emitting device may comprise a plurality of the first medium layers, the first medium layers may have a columnar shape, and be disposed in a grid, and the second medium layer may be disposed between the first medium layer. Further in this case, the grating may have a pitch in a periodic direction of (a+1) xcex/2n (xe2x80x9caxe2x80x9d is a positive integer and xe2x80x9cnxe2x80x9d is the mean refractive index).
(3) Further, at one of a hole transport layer and an electron transport layer may be provided.
In this case, the grating may have a single medium formed by the hole transport layer or the electron transport layer.
(4) The light-emitting layer may function as at least part of the grating.
(5) On at least one of the electrode layers, a layer regulating propagation of the light may be provided.
In this case, the layer regulating the propagation of light may be a cladding layer or a dielectric multilayer film.
(6) The light-emitting layer may be formed in a different region from the grating.
(7) The light-emitting layer may include an organic light-emitting material as a light-emitting material. By using an organic light-emitting material, compared with the use of for example a semiconductor material or inorganic material, the range of material available for selection is wider, and it is possible for light of various wavelengths to be emitted.
The above-described surface-emitting device of the present invention may be used for a display.
Examples are given of some materials that may be used in parts of the surface-emitting device of the present invention. These materials are only some well-known materials, and materials other than those cited by way of example can of course be selected.
The material of the light-emitting layer is selected from well known compounds for obtaining light of a particular wavelength. The material of the light-emitting layer may be either an organic compound or an inorganic compound, but from the point of view of large variety, and of ability to be formed into a film, can be an organic light-emitting material.
As such organic light-emitting materials can be used, for example, as disclosed in Japanese Patent Application Laid-Open No. 10-153967, aromatic diamine derivatives (TPD), oxydiazole derivatives (PBD), oxydiazole dimer (OXD-8), distyryl arylene derivatives (DSA), beryllium-benzoquinolinol complex (BeBq), triphenylamine derivatives (MTDATA), rubrene, quinacridone, triazole derivatives, polyphenylene, polyalkylfluorene, polyalkylthiophene, azomethine zinc complex, porphyrin zinc complex, benzoxazole zinc complex, phenanthroline-europium complex, and so on.
As the material of the light-emitting layer and may be used well-known materials such as those disclosed in Japanese Patent Application Laid-Open No. 63-70257, Japanese Patent Application Laid-Open No. 63-175860, Japanese Patent Application Laid-Open No. 2-135361, Japanese Patent Application Laid-Open No. 2-135359, Japanese Patent Application Laid-Open No. 3-152184, and further, Japanese Patent Application Laid-Open No. 8-248276 and Japanese Patent Application Laid-Open No. 10-153967. These compounds may be used singly or in combinations of two or more.
As examples of inorganic compounds may be cited ZnS:Mn (red region), ZnS:TbOF (green region), SrS:Cu, SrS:Ag, SrS:Ce (blue region), and so on may be cited.
When the light-emitting device section uses a light-emitting layer of an organic compound, if required, a hole transport layer can be provided between the electrode layer (anode) and light-emitting layer. The material used for the hole transport layer may be selected from well-known materials for injecting holes into a light propagating material, or well-known materials used in a hole injection layer in an organic surface-emitting device. The material of the hole transport layer has the function either of injecting holes or of obstructing electrons, and may be an organic or inorganic material. As a specific example, the material disclosed in, for example, Japanese Patent Application Laid-Open No. 8-248276 can be cited.
When the light-emitting device section uses a light-emitting layer of an organic compound, if required, an electron transport layer can be provided between the electrode layer (cathode) and light-emitting layer. The material of the electron transport layer should be such as to function to transport electrons injected from the cathode in the light-emitting layer, and this material can is be selected from well-known materials. As a specific example thereof, for example, can be cited the disclosure of Japanese Patent Application Laid-Open No. 8-248276.
As the cathode, a low work function (for example 4 eV or less) electron-injectability metal or metal alloy electrically conducting compound or combination thereof can be used. As such an electrode material, for example that disclosed in Japanese Patent Application Laid-Open No. 8-248276 can be used.
As the anode, a high work function (for example 4 eV or more) metal, metal alloy, electrically conducting compound or combination thereof can be used. When an optically transparent material is used as the anode, a transparent conducting material such as CuI, ITO, SnO2, ZnO, or the like can be used, and when transparency is not required, a metal such as gold or the like can be used.
In the present invention, the method of forming the grating is not particularly restricted, and a well-known method can be used. The following are typical examples.
(1) Method of Lithography
A positive or negative resist is exposed to ultraviolet radiation, X-rays, or the like, and developed, to pattern a resist layer, whereby the grating is created. As patterning technology using a resist of polymethylmethacrylate or novolac type resin or the like, for example Japanese Patent Application Laid-Open No. 6-224115, or Japanese Patent Application Laid-Open No. 7-20637 may be cited.
As technology for patterning by photolithography with polyimide may be cited, for example, Japanese Patent Application Laid-Open No. 7-181689 and Japanese Patent Application Laid-Open No. 1-221741. Further, as technology for using laser ablation, to form grating of polymethylmethacrylate or titanium oxide on a glass substrate may be cited, for example, Japanese Patent Application Laid-Open No. 10-59743.
(2) Method of Forming a Refractive Index Distribution by Light Irradiation
A grating is formed by irradiating light of a wavelength such as to cause variation in the refractive index in a light guiding member of an optical waveguide, and periodically forming portions of a different refractive index in the light guiding member. As such a method, in particular, a layer of a polymer or polymer precursor is formed, is partially polymerized by light irradiation or the like, and regions of a different refractive index are periodically formed, and thus a grating can be formed. As examples of this type of technology may be cited, for example, Japanese Patent Application Laid-Open No. 9-311238, Japanese Patent Application Laid-Open No. 9-178901, Japanese Patent Application Laid-Open No. 8-15506, Japanese Patent Application Laid-Open No. 5-297202, Japanese Patent Application Laid-Open No. 5-39480, Japanese Patent Application Laid-Open No. 9-211728, Japanese Patent Application Laid-Open No. 10-26702, Japanese Patent Application Laid-Open No. 10-8300, and Japanese Patent Application Laid-Open No. 2-51101.
(3) Method of Stamping
A grating is formed by stamping, for example using a thermoplastic resin for hot stamping (Japanese Patent Application Laid-Open No. 6-201907), stamping using an ultraviolet radiation cured type of resin (Japanese Patent Application Laid-Open No. 2000-35504), stamping using an electron beam cured type of resin (Japanese Patent Application Laid-Open No. 7-235075), and the like.
(4) Method of Etching
Using lithography and etching technology, a thin film is selectively removed to form a pattern, and a grating formed.
Above, methods of formation of a grating have been described, but it is sufficient that the grating is formed with at least two regions of mutually different refractive indices, and for example, can be formed by the method of forming two regions from two materials of different refractive indices, the method of forming two regions by modifying parts of a single material to form regions of different refractive indices, and so on.
The various layers of the surface-emitting device can be formed by well-known methods. For example, for the layers of the surface-emitting device, an appropriate method of film formation is selected depending on the material, and as specific examples may be cited vapor deposition, spin coating, the LB method, an inkjet method, and so on.